Numerical Analysis of Flow Structure Above Dimpled Surfaces with Different Depths in a Channel

Turbulent air flow characteristics in channels of three different dimple depth (δ/D = 0.1, 0.2, and 0.3) placed on the bottom wall are numerically predicted using FLUENT for Reynolds number based on the channel height, Re _H = 20,000, and the ratio of channel height to dimple print diameter, H/D = 1.0. The turbulence model employed is a realizable k ? ? model with no wall function. Steady-state analyses of fluid within and near different dimple depths demonstrate the existence of different vortex pairs. These vortex pairs are investigated at the central position of dimples as well as near the spanwise edge of individual dimples, and become stronger as dimple depth increases. In addition, steady-state results show that the streamwise vorticity distributions, which are associated with the flow recirculation zones positioned within the upstream halves of each dimple, are higher in magnitude and cover larger spatial extents as the dimple depth increases. Magnitudes of eddy diffusivity for momentum and eddy diffusivity for heat are also augmented by the vorticity concentrations within and downstream of individual dimples as the dimple depth increases. Such characteristics are due to the advection of reattaching and recirculating flow within the dimple cavities, as well as to the strong instantaneous secondary flows and mixing within the vortex pairs.     

Heat Transfer Enhancement by Jet Impingement on a Flat Surface with Detached-Ribs under Cross-flow Conditions

In this article, the heat transfer augmentation on a flat surface with jet impingement on axisymmetric detached ribs is numerically investigated. Both single and multiple jet impingement with and without crossflow interaction is investigated. Numerical simulations are done using Reynold-averaged Navier–Stokes (RANS) equations with a shear stress transport (SST) model with the commercial CFD code ANSYS-CFX. The influence of jet Reynolds number (7000 ≤ Re _j  ≤ 78,000), blowing ratio (5.8 ≤ M ≤ 11.5), and jet-outlet-to-target wall distance (2 ≤ H/D ≤ 6) are examined. Results show that the heat transfer is enhanced on the target wall with detached ribs in single and multiple jet impingement at moderate crossflow speeds.     

OPTIMUM DESIGN OF PLATE HEAT EXCHANGER WITH STAGGERED PIN ARRAYS

The optimization of the plate heat exchanger with staggered pin arrays for a fixed volume is performed numerically. The flow and thermal fields are assumed to be periodic fully developed flow and heat transfer with constant wall temperature and they are solved using the finite-volume method. The optimization is carried out by using the sequential linear programming (SLP) method, and the weighting method is adopted for solving the multiobjective problem. The results show that the optimal design variables for the weighting coefficient of 0.5 are as follows: S = 6.497 mm, P = 5.496 mm, D ₁ = 0.689 mm, and D ₂ = 2.396 mm. The Pareto optimal solutions are presented also.     

Numerical evaluation of the thermal performance of a solar air heater equipped with two different types of baffles

In the present study, the effect of utilizing two different types of baffles in the channel of the solar air heater is investigated numerically. The studied baffles include angled rectangular baffles and angled V‐shaped baffles, which are mounted on the bottom and top walls of the duct, respectively. Both considered baffles were evaluated separately which the studied parameter in each section was the angular position of baffles. Finally, the best‐obtained results of both sections were compared to each other. The results indicated that in the rectangular model by comparison between 90° model and no baffle, it was found that the pressure drop and average Nusselt number increase 316.67% and 148.15%, respectively at Reynolds number (Re) = 2000. Also, in V‐shaped angled baffles, the thermal efficiency of β = 90°, 60°, 45°, and 30° are 27%, 18%, 13% higher than no baffle channel at Re = 2000, respectively. Furthermore, at low Re (about Re below 300), utilizing baffles into the channel had no effect on the thermal efficiency of the system compared to the no baffle channel. However, at high Re, it was found that the highest thermal efficiency occurred in the model of rectangular baffles with an angle of 90°.     

Momentum and Heat Transfer Characteristics for the Flow of Power-Law Fluids over a Semicircular Cylinder

In this study, the two-dimensional steady flow of power-law fluids past a semicircular cylinder (flat face oriented upstream) has been investigated numerically. The governing equations (continuity, momentum, and energy) have been solved in the steady symmetric flow regime over the range of the Reynolds number (0.01 ≤ Re ≤ 25), power-law index (0.2 ≤ n ≤ 1.8), and Prandtl number (0.72 ≤ Pr ≤ 100). Extensive new results reported here endeavor to elucidate the role of power-law index (0.2 ≤ n ≤ 1.8) on the critical Reynolds number denoting the onset of flow separation (Re ^c ) and of vortex shedding (Re _c ). In shear-thinning fluids, both of these transitions are seen to be delayed than that in Newtonian and shear-thickening fluids. Furthermore, the influence of the Reynolds and Prandtl numbers, power-law index on drag phenomenon, and heat characteristics of semicircular cylinder have been studied in the steady flow regime. Finally, the present numerical values of the critical Reynolds numbers and the average Nusselt number have been correlated by simple forms which are convenient for interpolating these results for the intermediate values of the governing parameters in a new application.     

Heat transfer and fluid flow in shrouded pin fin arrays with and without tip clearance

Shrouded pin fin arrays with tip clearances (Cg) up to 25% of pin height were experimentally evaluated. Pressure loss was measured (2 × 10² < Re_D < 2 × 10⁴) and liquid crystal thermography was employed to obtain temperature distributions from which the impact of Cg on the mean heat transfer rate was determined for 2 × 10² < Re_D < 1 × 10⁴. Cg was found to influence pressure drop performance to the greatest extent at low Re_D, (<5 × 10³), with the effect being significantly diminished by Re_D = 1.5 × 10⁴. On a per unit pumping power basis, higher heat transfer rates were observed for dimensionless clearances (Cg/D) less than 0.2 as compared to the non-clearance case.     

Effects of Wall-Located Heat Barrier on Conjugate Conduction/Natural-Convection Heat Transfer and Fluid Flow in Enclosures

The effects of a heat barrier, located in the ceiling wall of an enclosure, on conjugate conduction/natural convection are investigated numerically. The vertical walls of the enclosure are differentially heated and the horizontal walls are adiabatic. Heatline technique is used to visualize heat transport. The variations of average Nusselt number, dimensionless heat transfer rate through the ceiling wall, and dimensionless overall heat transfer rate are studied. Calculations are performed for different Rayleigh numbers (10³ ≤ Ra ≤ 10⁶), thermal conductivity ratios (1 ≤ K ≤ 100), dimensionless locations of the heat barrier (0 < X _h  < 1),and two dimensionless ceiling wall thicknesses (D = 0.05 and D = 0.20). For high thermal conductivity ratio (K = 100), the heat barrier considerably reduces the dimensionless overall heat transfer rate. The effect of the heat barrier on dimensionless heat transfer rate through the enclosure increases as the Rayleigh number decreases. For low Rayleigh number (i.e., Ra = 10³), a location exists in the ceiling wall for which the dimensionless overall heat transfer rate is minimum.     

Investigation of flow behind vortex generators by stereo particle image velocimetry on a thick airfoil near stall

Stereoscopic Particle Image Velocimetry measurements investigating the effect of vortex generators (VGs) on the flow near stall were carried out in a purpose‐built wind tunnel for airfoil investigations on a DU 91‐W2‐250 profile. Measurements were conducted at Re = 0.9?10⁶, corresponding to free stream velocity U_∞ = 15 m s^?1. The objective was to investigate the flow structures induced by the vortex generators and study their separation controlling behavior on the airfoil. The uncontrolled flow (no VGs) displayed unsteady behavior with separation as observed from large streamwise velocity variations. The corresponding controlled flow (with VGs) showed the same unsteadiness, where the appearance of the vortex structures alternated with a much less separated or even attached boundary layer as also seen in the measured airfoil data: C_L = 1.56, C_D = 0.116 with VGs and C_L = 1.16, C_D = 0.135 without. On average, the controlled flow left an attached flow as opposed to the uncontrolled one. Mixing close to the wall, transferring high momentum fluid into the near wall region, was observed, and the hypothesis of variations in the streamwise velocity component in the boundary layer was supported by a Snapshot Proper Orthogonal Decomposition analysis. This analysis also revealed some of the dynamics of the induced vortices. Copyright © 2012 John Wiley & Sons, Ltd.     

MAXIMAL HEAT TRANSFER DENSITY IN VERTICAL MORPHING CHANNELS WITH NATURAL CONVECTION

In this article we show numerically that the entire flow geometry of a vertical diverging or converging channel with laminar natural convection can be optimized for maximal heat transfer rate density (total heat transfer rate per unit of flow system volume). The geometry is free to change in three ways: (1) the spacing between the walls, (2) the distribution of heating along the walls, and (3) the angle between the two walls. Numerical simulations cover the Rayleigh number range 10⁵ ≤ Ra_H ≤ 10⁷, where H is the channel height. Nonuniform wall heating is modeled as an isothermal patch of varying height H ₀ (≤H) on each wall, which is placed either at the bottom (entrance) end of the channel, or at the top (exit) end. The results confirm that the use of upper unheated sections enhances the chimney effect and the heat transfer. The new aspect is that the heat transfer rate density decreases because the unheated sections increase the total volume. It is shown that for maximal heat transfer rate density it is better to place the H ₀ sections at the channel entrance. It is also shown that the optimal angle between the two walls is approximately zero when Ra _H is large, i.e., for maximal heat transfer rate density the walls should be parallel or nearly parallel. Finally, the optimized spacing (1) developed in the presence of (2) and (3) as additional degrees of freedom is of the same order of magnitude as the optimal spacing reported earlier for parallel isothermal walls, i.e., in the absence of features (2) and (3). The robustness of the optimized flow architecture is discussed. Additional degrees of freedom and global objectives that may be incorporated in this constructal approach are the curvature of the facing walls and the mechanical strength and stiffness of the confining walls.     

Effects of Turbulent Reynolds Number on Turbulent Scalar Flux Modeling in Premixed Flames Using Reynolds-Averaged Navier–Stokes Simulations

The influence of turbulent Reynolds number (Re _t ) on the statistical behavior and modeling of turbulent scalar flux (TSF) has been analyzed using a direct numerical simulation database of freely propagating turbulent premixed flames. A range of different values of Re _t is considered in which the Damköhler and Karlovitz numbers are modified independent of each other to bring about the variation of Re _t . It has been found that the qualitative behavior of the various terms of the TSF transport equation does not change by the variation of Re _t , but their relative contributions to the transport of TSF are affected to some extent. The effects of Re _t on the modeling of the TSF using both algebraic and transport equation-based closures are addressed in detail. It is demonstrated that model parameters for an existing algebraic model, and the models for turbulent transport, pressure gradient and the reaction rate terms in the TSF transport equation, all exhibit Re _t dependence for small values of Re _t but all assume asymptotic values for Re _t  ≥ 50. By contrast, the model parameters for the combined molecular diffusion term are found to be insensitive to the variation of turbulent Re _t . Existing models for algebraic and transport equation-based closures of TSF have been modified to account for the observed Re _t dependence.     

An Investigation of Compressible Turbulent Forced Convection by an Implicit Turbulence Model for Large Eddy Simulation

An investigation of compressible turbulent forced convection in a three-dimensional channel flow is studied numerically by an implicit turbulence model for large eddy simulation (LES). Because of a high temperature difference between two walls and turbulent flow, the compressibility and viscosity of fluid should be taken into consideration simultaneously. Methods of the Roe scheme, preconditioning, and dual time stepping coordinating an implicit turbulence model for LES are used for resolving the effect of the compressibility of fluid on a low speed flow field. The magnitudes of Re _τ based on the friction velocity changing from 180 to 940, with the high temperature difference of two walls of 500 k are conducted. The results of the mean velocity profiles and turbulent intensities are in good agreement with the benchmark DNS data obtained by spectral codes from a low Reynolds number (Re _τ  = 180) to a high Reynolds number (Re _τ  = 940). Besides, the larger the Re _τ is, with the exception of acquirement of larger average Nusselt number, the more drastic variation of local instantaneous Nusselt number is observed.     

Numerical Study of Sphere Drag Coefficient in Turbulent Flow at Low Reynolds Number

The drag coefficient of a sphere immersed in turbulent air flow in the Reynolds number (Re = U _∞ d/ν _∞) range up to 250 and turbulence intensity (u _∞′/U _∞) up to 60% is computed numerically. Reynolds-averaged Navier-Stokes equations (RANS) are solved in Cartesian coordinates by using a blocked-off technique. To our knowledge, the present work is the first to employ the blocked-off technique for flow over a sphere. Closure for the turbulence stress term is accomplished by testing four different turbulence closure models. The main findings of the present investigation are that the laminar numerical data compare well with numerical and experimental published work. However, different turbulence closure models produce different trends in the range of Reynolds number up to Re = 100, and this difference is demarcated by the nonagreement between the turbulent predictions and the “standard” drag coefficient results. However, the results obtained using Menter's SST turbulence model show fair agreement with the well-known sphere “standard” drag over the range of test conditions explored here. Thus, the present results confirm recently published findings, which suggest that the free-stream turbulence intensity does not have a significant effect on the sphere mean drag.     

Comparison of External Surface Heat Transfer Coefficients for Circular and Oval Tubes

Experimental measurements were carried out to compare the air-side heat transfer coefficients of three oval tubes (axis ratio 2, 3, and 4) with those of an equivalent circular tube (o. d. 18 mm). The tubes were tested as single tubes in a cross-flow of air. The range of the investigated Reynolds numbers Re _D was between 1000 and 11000. The effects of the area blockage and the free stream turbulence were taken into consideration in the evaluation of the thermal performance. The measurement results indicate that Nusselt numbers Nu _D for the tested circular and oval tubes are close at the lower range of the tested Reynolds numbers (Re _D < 4000) corresponding to an air velocity < 4 ms ^?1 in this work, which is the air velocity for most air conditioning applications. For Re _D > 4000, the Nu _D for the circular tube are higher than those for the oval tubes, and the Nu _D for the oval tubes decrease with the increase of the axis ratio.     

Flow Characteristics Behind Rectangular Cylinder Placed Near a Wall

Three-dimensional unsteady flow over a bluff body located parallel to a wall, kept at different gap height from the wall, has been studied numerically. The bluff body considered is a rectangular cylinder with two different aspect ratios, B/D = 1 and B/D = 2, where B and D are the width and height of the cylinder, respectively. The flow is considered as a laminar flow, and the Reynolds number based on the height of the cylinder cross section and oncoming reference velocity is 450. Numerical study is carried out by varying the distance of the cylinder from the wall, and the development of the vortex shedding phenomenon under the influence of the wall is investigated. From previous experiments, it is observed that as the distance between the wall and the cylinder decreases, the wake behind the cylinder becomes stationary and the vortex shedding is suppressed. The present numerical study confirms a similar trend. Periodic activity in the downstream of the flow is disturbed completely with decreasing gap between the wall and the cylinder.     

Investigation of dimpled fins for heat transfer enhancement in compact heat exchangers

Direct and Large-Eddy simulations are conducted in a fin bank with dimples and protrusions over a Reynolds number range of Re_H = 200 to 15,000, encompassing laminar, transitional and fully turbulent regimes. Two dimple-protrusion geometries are studied in which the same imprint pattern is investigated for two different channel heights or fin pitches, Case 1 with twice the fin pitch of Case 2. The smaller fin pitch configuration (Case 2) develops flow instabilities at Re_H = 450, whereas Case 1 undergoes transition at Re_H = 900. Case 2, exhibits higher Nusselt numbers and friction coefficients in the low Reynolds number regime before Case 1 transitions to turbulence, after which, the differences between the two decreases considerably in the fully turbulent regime. Vorticity generated within the dimple cavity and at the dimple rim contribute substantially to heat transfer augmentation on the dimple side, whereas flow impingement and acceleration between protrusions contribute substantially on the protrusion side. While friction drag dominates losses in Case 1 at low Reynolds numbers, both form and friction drag contributed equally in Case 2. As the Reynolds number increases to fully turbulent flow, form drag dominates in both cases, contributing about 80% to the total losses. While both geometries are viable and competitive with other augmentation surfaces in the turbulent regime, Case 2 with larger feature sizes with respect to the fin pitch is more appropriate in the low Reynolds number regime Re_H < 2000, which makes up most of the operating range of typical compact heat exchangers.     

An experimental study of heat transfer in oscillating flow through a channel filled with an aluminum foam

An experimental study has been conducted on the heat transfer of oscillating flow through a channel filled with aluminum foam subjected to a constant wall heat flux. The surface temperature distribution on the wall, velocity of flow through porous channel and pressure drop across the test section were measured. The characteristics of pressure drop, the effects of the dimensionless amplitude of displacement and dimensionless frequency of oscillating flow on heat transfer in porous channel were analyzed. The results revealed that the heat transfer in oscillating flow is significantly enhanced by employing porous media in a plate channel. The cycle-averaged local Nusselt number increases with both the kinetic Reynolds number Re_ω and the dimensionless amplitude of flow displacement A₀. The length-averaged Nusselt number is effectively increased by increasing the kinetic Reynolds number from 178 to 874 for A₀ = 3.1-4.1. Based on the experimental data, a correlation equation of the length-averaged Nusselt number with the dimensionless parameters of Re_ω and A₀ is obtained for a porous channel with L/D_h = 3.     

Pressure Loss Through Sharp 180 deg Turn in a Relatively Short Two－Pass Smooth and Rib－Roughened Channel

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Evolutionary Placement of Discrete Heaters in Forced Convection

This article aims to maximize the global conductance (C) of a symmetrical, discretely heated channel in forced convection, where the fluid flow is sustained by a fixed pressure difference given by the Bejan number. The maximization of C is obtained by determining the optimal arrangement of the discrete heaters along the channel and the optimal channel breadth with the help of a genetic algorithm (GA) that is fully coupled with the finite-element methods used for solving the conservation equations. The number of independent variables considered in the optimization process varies between N + 1 and 2N + 1, where N is the number of heat sources (1 ≤ N ≤ 20) and the extra unit represents the channel height. The numerical results agree with the available literature, showing that increased values of C are obtained with designs that do not use equally spaced heaters. The results show that a larger number of discrete heaters can provide higher values of global conductance when compared with fully optimized simpler designs (i.e., a small number of discrete heaters), which is also in agreement with previous studies. Designs with heaters of variable heat strength are also considered, to study the optimal allocation of the total heat input through the N heaters. This family of designs leads to even higher performance.     

Effects of thermal property variations on the liquid flow and heat transfer in microchannel heat sinks

Three-dimensional conjugate numerical simulations using the inlet, average and variable thermal properties respectively were performed for the laminar water flow and heat transfer in rectangular microchannels with D_h of 0.333 mm at Re of 101–1775. Both average and variable properties are adopted in data reduction. The calculated local and average characteristics of flow and heat transfer are compared among different methods, and with the experiments, correlations and simplified theoretical solution data from published literatures. Compared with the inlet property method, both average and variable property methods have significantly lower f_app, but higher convective heat transfer coefficient h_z and Nu_z. Compared with the average property method, the variable property method has higher f_appRe_ave and lower h_z at the beginning, but lower f_appRe_ave and higher h_z at the later section of the channel. The calculated Nu_ave agree well with the Sieder-Tate correlation and the recently reported experiment, validating the traditional macroscale theory in predicting the flow and heat transfer characteristics in the dimension and Re range of the present work.